I have a Tokina AT-X-Pro 28-70mm F2.6-2.8 lens and I can't seem to get the lens down to f2.6 (f2.8 seems to be the maximum), either on program or manual on my Nikon D700. Is there a trick to get to these unusual aperture settings?

I should just add that even changing the aperture on the lens barrel to f2.6 the camera still thinks it is f2.8

Thanks for any help

Dave